For example, a liquefied natural gas (hereinafter simply referred to as LNG) is generated by first precooling a natural gas at room temperature under normal pressure to about −30° C., further cooling the resulting gas for liquefaction, and then further supercooling it to −162° C. This cooling process employs refrigeration units using various refrigerants. Each refrigeration unit has a compressor, a condenser, a throttle expansion unit, and an evaporator connected in sequence in a refrigerant path, thereby forming a closed refrigerating cycle.
PTLs 1 to 5 each disclose a liquefied gas cooling apparatus for an LNG and the like, using a refrigeration unit as described above. These liquefied gas cooling apparatuses each include different refrigeration units having needed performances for a precooling process and a liquefaction process.